Best processor: how to buy the right CPU for you

Time for a new CPU? Here's how to get the right one

Whether you're buying a new PC, or upgrading an old one, perhaps with a new motherboard, then your choice of PC processor will be critical. But there's a lot to consider if you want to get the best processor for your PC.

So what's the best CPU for you? Will you opt for AMD, or Intel, for instance? How much performance do you really need? How many CPU cores will it take to achieve that? What might you be able to achieve for overclocking? And how much is all this going to cost? Which processor is best for gaming? What's the best processor for Photoshop users? The list goes on, and on.

Fortunately, while there's a huge list of processors available, it's actually not that difficult to reduce the selection to more practical and manageable levels to decide which processor to buy. You just have to take the process step by step, analysing your needs and looking at the best candidates to fulfil them, and this begins by thinking about the age-old question: Intel or AMD?

Intel vs AMD

You might well already have chosen your preferred CPU manufacturer, of course. Maybe you always go with one particular company. Or perhaps you have your motherboard already, in which case the decision has been made already: you'll have to opt for whatever is compatible.

If not, though, selecting Intel or AMD will pose an interesting dilemma.

Right now, Intel is the performance king. Its Core i7 range is blisteringly fast and, initially at least, outperform anything that AMD has to offer.

This comes at a price, though - if you're looking for value, then AMD offers a much better deal. As we write, for instance, the quad-core AMD Black Edition Phenom II X4 965, one of AMD's fastest processors, can be yours for around £130. Intel's core i5-680 delivers roughly equivalent performance, but will cost you maybe £230. And becoming a speed king via something like Intel's Core i7-950 will virtually double the figure again: you can expect to pay £450 or more.

(These figures change day to day, but AMD will remain the value choice for the foreseeable future. If you already have a few CPUs in mind, check our processor reviews and more up-to-date price comparisons.)

It's also worth factoring in the price of the motherboard. Spending just £60 to £70 will give you plenty of possible homes for a high-end AMD CPU; opt for the best that Intel has to offer and you'll probably spend at least twice as much. (But again, visit our motherboard reviews for up-to-date prices.)

If you want and need Intel's performance then things aren't quite as bad as they seem. Intel CPUs are generally a little more overclockable than AMDs, and something like the i5-750 (priced around £150 as we write) can be pushed a very long way.

But if you don't require that kind of high-end power (or you just can't afford it) then an AMD processor is the way to go. You'll still get plenty of speed when you need it, and will save a pile of cash, too.

These days the CPU features that attract the most attention are the number of cores, and its clock speed. Both are simple concepts to understand, but you still have to be careful how you interpret them.

AMD's Black Edition Phenom II X6 1090T, for example, contains six cores, individual processors, which means the CPU can work on six separate tasks at the same time. And each of these runs at a very respectable 3.2GHz.

Meanwhile the Intel Core i7-860 has only four cores, and runs at a mere 2.8GHz. So you might, not unreasonably, expect it to be slower. However, it supports Intel's Hyper-Threading technology, providing an extra four "virtual" cores, and thanks to this, and a few other design decisions, it actually outperforms the Phenom II X6 1090T. So you can't rely solely on these figures to determine which CPU is best.

Still, as a general rule for processors from the same manufacturer, the more cores a CPU provides, the happier you're going to be. You won't see large speed increases from all programs - the popular LAME encoder is still single-threaded, for instance, so upgrading from a dual to quad-core CPU won't deliver any noticeable changes - but most apps that could benefit significantly from multithreading, now do so, and will take advantage of the extra processing power. And just about everything else will be upgraded over time.

Cores should be your first priority, then: you'll want a quad-core CPU at a minimum. After that, opt for the highest clock speed you can afford, or need, but don't worry about that quite as much. You can often ramp that up later with a little judicious overclocking.

Every processor comes sized to fit a particular "CPU socket", a component that connects the CPU to a motherboard. If you've chosen your motherboard already then you'll know what this is; if not, then now might be a good time to think about it, as your processor will need to be compatible.

If you're opting for an AMD CPU then there a couple of options. The cheapest is to go for a Socket AM2+ motherboard, but choosing Socket AM3 instead will get you support for faster DDR3 RAM. AM3 boards will typically also come with USB 3.0 and SATA 6 Gbps, and prices are so affordable that it makes little sense to go for anything else.

If you're choosing an Intel Core i5 CPU then there's even less to think about: you'll want an LGA1156 motherboard.

But if you're splashing out on a Core i7 then it's a little more complicated.

You could buy a Core i7 CPU that also fits the LGA1156 motherboard. These are known as the 800 series, so they'll all have product numbers that are 800 and something: the Core i7-860, say. This will save you a little money, but the LGA1156 boards have some limitations, and for example are often lacking in PCI Express lanes, cutting your expansion possibilities.

The alternative is to buy a Core i7 CPU designed for LGA1366 motherboards, which means 900 series chips like the Core i7-930 or -940. Not only is the board more powerful, but it's also reasonably future-proof. High-end Intel releases like the 6-core i7-975 are already available in the LGA1366 format only, for instance.

If you're building a power system, need plenty of PCI Express power (perhaps to install multiple graphics cards) or want to ensure your system's future upgradeability, then, an LGA1366 board is the way to go.

But if a Core i7 CPU already feels more expensive than you'd really like to pay, and you're just building a regular system, then an LGA1156 board and processor will be fine. Avoid the PCI Express issues and there's little difference in performance, and you can put the money you save towards enhancing another area of your PC.

CHOOSE WISELY: Your choice of motherboard will define exactly which CPUs you can install

We've reduced the original mountain of possible CPUs to just a handful, then, but to choose the right model from these you'll need to understand just a few more processor-related terms.

As we've mentioned, many Intel CPUs include a feature called Hyper-Threading, which allows each core to run two sets of instructions simultaneously. It can be a very effective technology - Intel Core i7 CPUs are essentially i5's with Hyper-Threading enabled - and you'll want to get it if you can.

Every Intel processor also contains an amount of embedded memory called a cache, which is used to hold regularly-used information. If the CPU needs this again then it can be fetched from the cache, which is far quicker than accessing system RAM. Look for descriptions like L2 (Level 2 Cache), L3 (Level 3 Cache) or "Intel Smart Cache" (which means L3) - the more your processor has, the better.

And it's a similar story with AMD CPUs, although less dramatic. Opt for a quad-core Phenom II X4 and you'll find they all have the same cache amounts, 2MB for L2, 6MB for L3. The 6-core Phenom II X6 increases L2 cache to 3MB, though, while will provide a useful speed increase in many situations.

What you might want also to consider, though, is the Phenom II's "Thermal Design Power" (TDP), the maximum amount of power it's likely to consume when busy running applications. This ranges from 65W (Phenom II X4 905e or 910e) to 125W (Phenom II X4 955 and higher); the low TDP CPUs are slower, but use less electricity and run cooler, making them better choices for media centre PCs where you want to keep fan noise to a minimum.

You should now have a better idea of what you want from a CPU, then - but you'll still need to find confirmation of exactly which model is best for you. And our processor reviews are a great place to start. There are over 100 reviews online, all filterable by socket, CPU type and more, and with instant price comparisons to hand you'll soon locate the best CPU deals around.

NOT THE BEST:It's a great CPU, but the Intel Core i5 750 lacks Hyper-Threading, reducing its performance a little

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